The present invention relates to a method for assessing the remaining lifespan of a bolt used at high temperatures. In particular, the present invention relates to a method for assessing the remaining lifespan of a bolt appropriate for assessing the remaining lifespan of a bolt used at high temperatures by detecting deteriorated conditions in a non-destructive manner. Examples of such bolt include a bolt for a turbine pipe, a turbine valve and a turbine casing.
In general, rotor shafts, casings, casing bolts, and the like that constitute turbines are produced with the use of CrMoV-based low alloy steels, 12Cr steel ferritic stainless steels, and the like. In general, when such types of high-temperature members are subjected to stress loading for many hours in a high temperature atmosphere ranging from approximately 300° C. to 600° C., a carbide precipitates at a crystal grain boundary or in a crystal grain or void formation takes place at a grain boundary, resulting in deterioration of the members. Such deterioration might cause generation of cracks on a member constituting a turbine, eventually leading to a destructive turbine accident. In order to prevent such accident or to operate a turbine in an economical manner, it is important to correctly asses the lifespan of turbine parts.
In a conventional method for examining damaged conditions of a high-temperature member exposed to a high temperature atmosphere for many hours, a test piece is directly excised from a member that has been used in practice and the test piece is subjected to a destructive test. However, in recent years, an assessment method involving non-destructive testing has been used. In such case, damaged conditions can be evaluated while allowing a member to remain in an installed state.
For instance, JP Patent Publication (Kokai) No. 2-28554 A (1990) discloses a damage detection method for a high-temperature apparatus, comprising determining the shapes of microscopical defects that are generated in the tissue of a heat-resistant alloy used at high temperatures and comparing the results with predetermined quantitative damage based on the correlation between the microscopical defects and the quantitative damage so as to detect actual damage. However, for instance, in the cases of high Cr ferritic heat-resistant steels, which have been often used for high-temperature members in recent years, there are few tissue changes caused by creep damage and thus void formation is unlikely to take place at a grain boundary. Therefore, it is very difficult to assess remaining lifespan by observing microscopical defects.
JP Patent Publication (Kokai) No. 2003-270220 A (2003) discloses a lifespan prediction method, comprising irradiating a high-temperature member to be assessed with ultrasonic waves, determining the sonic velocity value thereof, and comparing the obtained sonic velocity value with that derived from an unused material and with that derived from a member subjected to load-free heat treatment, thereby calculating the remaining lifespan of the member. JP Patent Publication (Kokai) No. 58-92952 A (1983) discloses a method for predicting the lifespan of a high-temperature member based on the relationship between creep strain and decrease in hardness of a metallic material portion used at high temperatures. However, the above lifespan prediction methods are not satisfactory in terms of prediction precision.